Ferromagnetic materials, such as iron, nickel, steel and other materials, are used to make many structures, such as beams in buildings, pipes, parts of machinery or vehicles, and ocean vessel hulls, to list a few examples. As used herein, “ferromagnetic materials” include both ferromagnetic and ferrimagnetic materials. These materials have hysteretic properties, which allow them to retain residual magnetic fields and to become permanently magnetized. As used herein, “permanently magnetized” refers to aligning magnetic domains to create an internal residual field which remains without the presence of any external field. In many cases, these materials are subject to fatigue, corrosion and/or erosion. As used herein, corrosion means loss of material as a result of chemical reaction, most commonly oxidation. As used herein, erosion means loss of material as a result of a mechanical process, such as abrasion. For example, sand produced in oil or gas wells can abrade the inside of a pipeline carrying oil or gas from the well. Material loss due to corrosion and/or erosion is collectively referred to herein as a “defect.” As used herein, the term defect also includes a crack, or a void or inclusion of foreign material, such as might occur during manufacture or later. In addition, defects can also occur due to fatigue and wear. If allowed to proceed beyond a critical point, defects may lead to catastrophic failures such as collapse or an oil spill.
Visual inspection for defects in ferromagnetic structures is typically not practical for structures that are hidden from sight. For those instances where visual inspection is practical it will find the most obvious damages not the subtle ones. Therefore, various apparatus and non-visual methods have been used in the prior art in attempts to detect defects in ferromagnetic structures and items made of ferromagnetic materials. Some of these apparatus and methods require removing thermal insulation and striping off corrosion inhibiting surface treatments to gain direct access to a surface of the ferromagnetic material. In some cases, the surface must be polished to create a pristine interface to a sensor or wave propagation from the sensor. These steps are costly, time-consuming and often compromise the thermal insulation and/or the surface treatments.
Some prior art apparatus and methods involve magnetometry in attempts to detect defects in ferromagnetic materials. For example, U.S. Pat. Nos. 8,542,127 and 8,447,532, both by Valerian Goroshevskiy, et al., disclose using the inverse magnetostrictive Villari effect. The inverse magnetostrictive Villari effect involves changes in a material's magnetic susceptibility under applied mechanical stress. If a structure suffers a defect, the structure's magnetic susceptibility when the structure material is mechanically stressed, for example when the structure is pressurized, is different than when the structure is not mechanically stressed. The Goroshevskiy patents rely on detecting this change in magnetic susceptibility as pressure within the structure changes. Thus, energy must be introduced into the structure in the form of pressurizing the inferior of the structure. Some structures remain unused, and therefore unpressurized, for periods of time during which defects may develop. Other structures, such as ship hulls or structural elements, do not lend themselves to known pressurization cycling. However, without pressurization, the Goroshevskiy apparatus and methods cannot detect these defects. Furthermore, Goroshevskiy can determine a defect's location only along the length of a structure; Goroshevskiy cannot determine the defect's location circumferentially around the pipe.
Two more recent patents, U.S. Pat. Nos. 9,651,472 and 9,651,471, disclose characterizing defects using magnetic flux leakage (MFL) from ferromagnetic pipes. These patents describe an approach in which arrays of magnetometers disposed about a surface of the ferromagnetic material to sense its magnetic field. This yields magnetic flux data that can be rendered into two-dimensional maps. A pattern matcher can then be used to identity defects.
MFL technique is an established technique. For example, see Miller, “Prediction of Dent Size using Tri-axial Magnetic Flux Leakage Intelligent Pigs”, Document Id: NACE-07138, NACE International, 2007. Note that PIG is an acronym for Pipeline Inspection Gauge. Miller's review article points out use of MFL techniques that primarily collect data on-skin or near-skin from defects on ferromagnetic pipes.
Transkor Group, Inc., from Houston, Tex., and Energodiagnostika, a Russian company, have developed methods, classified as large standoff magnetometry (LSM) for passively measuring the magnetic flux leakage of defects at large standoff distances.